Tweak library load path insertion to be as early as possible

Library/.rubocop.yml

@@ -202,8 +202,10 @@ Performance/MethodObjectAsBlock:
 Rails:
   # Selectively enable what we want.
   Enabled: false
-  # Do not use ActiveSupport in RuboCops.
   Exclude:
+    # This file is loaded before any extra methods are defined.
+    - "Homebrew/standalone/init.rb"
+    # Do not use ActiveSupport in RuboCops.
     - "Homebrew/rubocops/**/*"
 
 # These relate to ActiveSupport and not other parts of Rails.

Library/Homebrew/standalone/init.rb

@@ -39,7 +39,13 @@ if !gems_vendored && !ENV["HOMEBREW_SKIP_INITIAL_GEM_INSTALL"]
   ENV["HOMEBREW_SKIP_INITIAL_GEM_INSTALL"] = "1"
 end
 
-$LOAD_PATH.unshift HOMEBREW_LIBRARY_PATH.to_s unless $LOAD_PATH.include?(HOMEBREW_LIBRARY_PATH.to_s)
+unless $LOAD_PATH.include?(HOMEBREW_LIBRARY_PATH.to_s)
+  # Insert the path after any existing Homebrew paths (e.g. those inserted by tests and parent processes)
+  last_homebrew_path_idx = $LOAD_PATH.rindex do |path|
+    path.start_with?(HOMEBREW_LIBRARY_PATH.to_s) && !path.include?("vendor/portable-ruby")
+  end || -1
+  $LOAD_PATH.insert(last_homebrew_path_idx + 1, HOMEBREW_LIBRARY_PATH.to_s)
+end
 require_relative "../vendor/bundle/bundler/setup"
 $LOAD_PATH.unshift "#{HOMEBREW_LIBRARY_PATH}/vendor/bundle/#{RUBY_ENGINE}/#{Gem.ruby_api_version}/gems/" \
                    "bundler-#{Homebrew::HOMEBREW_BUNDLER_VERSION}/lib"

Library/Homebrew/test/spec_helper.rb

@@ -33,7 +33,7 @@ require "find"
 require "byebug"
 require "timeout"
 
-$LOAD_PATH.push(File.expand_path("#{ENV.fetch("HOMEBREW_LIBRARY")}/Homebrew/test/support/lib"))
+$LOAD_PATH.unshift(File.expand_path("#{ENV.fetch("HOMEBREW_LIBRARY")}/Homebrew/test/support/lib"))
 
 require_relative "../global"